Predicting global killer whale population collapse from PCB pollution

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Science  28 Sep 2018:
Vol. 361, Issue 6409, pp. 1373-1376
DOI: 10.1126/science.aat1953

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  • RE: A final comment
    • Lars Witting, Senior Scientist, Greenland Institute of Natural Resources

    In their response, Deforges et al. (2019) recognise the limitations of their model approach, but the fundamental problems with the Deforges et al. (2018) paper remain. Instead of recognizing the limitations of the approach, the paper promotes a bold prediction about killer population collapses. The predicted global decline is not a realistic median projection, and the paper don’t explore the likely range of potential growth (2, 3 or 4% or even higher). Instead, it assumes an unsubstantiated pristine growth rate of 1% or less. This unrealistically low value is a main cause for the fake prediction of Deforges et al. (2018). An empirically driven risk assessment is better than having no assessment at all only when supported by data, and we would all be better off if the paper of Deforges et al. (2018) is withdrawn.

    Two extra comments:

    1) Deforges et al. (2019) report that the baseline growth rate of their model is 1.01%; but the growth of the pristine projection in Deforges et al. (2018) is their reported 141% increase over 100 years, producing an annual exponential growth rate of r = ln (2.41^0.01) = 0.88%.

    2) My rebuttal does not state, as claimed by Deforges et al. (2019), that the observed growth of a pristine population varied from -0.5 to 4%. This range is a reported range for the within population variation in the growth rate between interacting groups, and it can thus not be taken as a realistic range for the growth of a population, as claimed by...

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    Competing Interests: None declared.
  • RE: Response to Witting’s, and Elliott & Trites’ E-Letters

    Our recent Science Report, “Predicting global killer whale (Orcinus orca) population collapse from PCB pollution” (1) has received much attention, including critical eLetters by Witting and Elliott & Trites. While specific criticisms of our paper will be addressed below, we believe it is important to take a step back and consider again the broad purpose of our study. Given the almost complete lack of knowledge of PCB effects at the population-level in killer whales, coupled with extremely elevated concentrations of these chemicals and decades of studies providing empirical evidence of their toxicity to a broad variety of mammalian species, our goal was to provide the first global assessment of the potential risk of PCB exposure in killer whales. We certainly acknowledge the limitations of our model approach (as discussed below), but strongly feel that having an empirically driven risk assessment based on the best available data is far better than having no assessment at all. In other words, how can policy makers make informed decisions about threats to killer whales if the various threats have not been investigated? We believe our paper achieved this goal, and we now embrace the wider discussion of the topic as it clearly demonstrates the widespread public and scientific interest in the conservation of this flagship marine species.

    Several comments by L. Witting and Elliott & Trites focused on key assumptions and parameters of our model design. To be clear,...

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    Competing Interests: None declared.
  • RE: Predicting global killer whale population collapse from PCB pollution
    • Tony R Walker, Dalhousie University
    • Other Contributors:
      • Priyanka Varkey

    Slow motion extinction of Southern Resident Killer Whales

    There are now only 74 Southern Resident Killer Whales (SRKWs). We are witnessing a slow-motion SRKW extinction. Since 1998, only 40 SRKW calves have survived and no newborn whales have survived since 2015. The SRKW population is the only orca population listed as endangered by the Canadian Species at Risk Act (SARA) (1). SRKWs are found from Alaska to California and feed almost exclusively on Chinook salmon. Three main stressors to this dwindling population are noise pollution from increased vessel traffic, ocean contaminants, and declines in Chinook salmon (2). Vessel noise affects social communication and echolocation in orcas (3). SRKWs rely on echolocation for navigation and hunting. Difficulty in finding prey is amplified by lack of Chinook salmon combined with vessel noise (1, 2). The Haro Strait off the coast of British Columbia, Canada is one of the loudest areas along the Pacific Coast and is the summertime feeding habitat of SRKWs. Reducing vessel speed to 11.8 kt can reduce underwater vessel noise by 3 dB.

    Fisheries and Oceans Canada (DFO) and the United States National Oceanic and Atmospheric Administration (NOAA) have implemented regulations attempting to mitigate the stress on the orca population. DFO is working with U.S. agencies to coordinate measures to reduce underwater noise impacts on the SRKWs (4). In 2017, DFO Canada published an action plan that outlines measures to aid in the r...

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    Competing Interests: None declared.
  • RE: PCBs to decline, not killer whales
    • John Elliott, Research Scientist, Environment and Climate Change Canada
    • Other Contributors:
      • Andrew Trites, Professor, University of British Columbia

    Desforges et al. (28 Sep, p 1373–1376) predict that exposure to PCBs will cause an apocalyptic global collapse of killer whale populations residing near industrialized regions over the next 100 years. However, their contention is not supported by data both on PCB persistence and killer whale dynamics along the west coast of North America. Erroneous conclusions about the demise of killer whales not only demean the substantial efforts made in North America to manage PCB contamination (1), but also distract from the more important problems facing killer whales of increased shipping, underwater noise, and threats to their food supplies (2, 3).
    PCB concentrations have declined precipitously in monitored food chains since the late 1970s, including in the Salish Sea where fish-eating southern resident killer whales (SKRWs) and marine mammal-eating transient killer whales (TKW) regularly feed. Total PCBs, for example, in eggs of resident double-crested cormorants were 25-fold lower in 2002 than in 1970 (4)—similar to PCB declines reported elsewhere (5-8).
    The SRKW and TKW populations should have been decimated during the 1970s if the dose-response relationships used by the authors between PCBs, reproduction, and immunotoxicity were realistic. However, SRKWs increased by 33% (from 1973 to 1995), and the TKWs more than quadrupled (from the late 1970s to present). TKWs have increased by ~3% per year since the early 1980s despite having 3-20X greater PCB loads than the l...

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    Competing Interests: None declared.
  • Saving killer whale populations from a global collapse
    • Lars Witting, Senior Scientist, Greenland Institute of Natural Resources

    Desforges et al. (2018) address PCB pollution in killer whales, predicting a decline in calf survival and an associated population collapse worldwide. Witting (2018) refutes the collapse showing that it follows from a flawed model parametrisation.

    Desforges et al. (2018) assume that pristine non-polluted killer whale populations can increase at about 0.9% per year. This is unrealistically low because the extensive data of Olesiuk et al. (1990) and Matkin et al. (2014) have shown that killer whale populations that are exposed to small concentrations of PCB can increase at rates between 2.9% and 4.2% per year.

    Assuming a somewhat conservative pristine growth rate from 2.9% to 3.4%, Witting (2018) corrected the analysis to find that killer whales across all pollution levels in Desforges et al. (2018) have the potential for positive growth. There is absolutely no evidence for a global PCB driven collapse of killer whale populations. Yet, the density regulation of the most exposed killer whale populations may compensate only partly for the detrimental effects of PCB.


    Desforges, J.-P., A. Hall, B. McConnell, A. Rosing-Asvid, J. L. Barber, A. Brownlow, S. De Guise, I. Eullaers, P. D. Jepson, R. J. Letcher, M. Levi, P. S. Ross, F. Samarra, G. Vikingson, C. Sonne and R. Dietz 2018. Predicting global killer whale population collapse from PCB pollution. Science 361:1373–1376.

    Matkin, C. O., J. W. Testa, G. M. Ellis and E. L. Saulitis 2014...

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    Competing Interests: None declared.
  • Orcas and PCBs
    • Giovanni Di Guardo, Associate Professor, University of Teramo, Faculty of Veterinary Medicine, Teramo, Italy
    • Other Contributors:
      • Antonio Fernández, Full Professor, Universidad de Las Palmas, Instituto Universitario de Sanidad Animal y Seguridad Alimentaria, Las Palmas, Gran Canaria, Spain

    The dramatic population decline which has been predicted to affect killer whales (Orcinus orca) on a global scale by the end of this century is of concern, with the levels of polychlorinated biphenyls (PCBs) in tissues from free-ranging orcas having been estimated to be among the highest in the animal kingdom (1).
    As in other cetacean and non-cetacean “top predators”, in fact, lipophilic PCBs may heavily accumulate in killer whales’ subcutaneous blubber, thereafter undergoing ad hoc “biomagnification” processes. Since these bioaccumulation and biomagnification dynamics are shared by many other persistent environmental pollutants - either “classical” (i.e. DDTs, dioxins, heavy metals, etc.) or “emerging” (i.e. PBDEs, PFAS, micro- and nanoplastics, etc.) -, that almost unvariably form “mixtures” to which aquatic organisms are chronically exposed via the marine food web(s) (2), one could wonder how the biological effects of PCBs may be effectively “dissected” from those of the other contaminants present in the aforementioned mixtures. Furthermore, the “endocrine disrupting” and the additional pathogenic activities of PCBs on host’s reproductive and immune functions are also known to be exerted by other organochlorine (OC) pollutants, the entry of which into exposed animals’ cells is mediated by aryl hydrocarbon receptors (AHRs) (3). This implies that the susceptibility of a given species to PCBs and, more in general, to OC contaminants could “ideally” result from the “s...

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    Competing Interests: None declared.